This invention relates to heating electrically conductive material, such as metal strip, by transverse flux induction, or xe2x80x9cTFIxe2x80x9d. By way of example, such heating can be for the purpose of affecting the metal itself or for the purpose of affecting coatings on the metal.
Background information on TFI is presented in the article xe2x80x9cInduction heating of strip: Solenoidal and transverse fluxxe2x80x9d by Nicholas V. Ross and Gerald J. Jackson, IRON and STEEL ENGINEER, September 1991.
TFI heating of metal strip can over-heat the edges of the strip, when the inductor coil is wider than the strip. This can occur due to electromagnetic phenomena at the discontinuity in electrical conduction formed at an edge. See FIG. 6 of the article referenced in the previous paragraph. At metal locations removed from the edge, electrical current density may be low, while, at the edge, the same current can be forced into a very limited region, thereby greatly increasing current density, leading to over-heating and, particularly in the case of aluminum, even to edge melting.
It is an object of the invention to provide new methods and installations of TFI characterized by the ability to deliver significantly reduced amounts of electrical current and current density to edge regions of electrically conductive material, such as metal strip, compared to that delivered across the width of the material.
The invention provides a number of improvements in the arrangement of the coils of the inductors for TFI heating of electrically conductive material, such as metal strip, or graphite cloth. For instance, coil conductors that cross the strip width are stacked, or connected, such that a multiple of the induced current flows across the strip width as compared to that which flows along the strip edges. Alternatively, or additionally, by shaping the conductors to a wedge, or other concentrating shape, we can induce currents in the strip within a narrow width, in order to increase the current density across the strip width compared to that which flows along the strip edge. Alternatively or additionally, the coils have variable dimensions, in order to adjust the inductive heating effect.
Preferably, the coil conductors across the strip width and the coil conductors along the strip edges are connected in series to insure that the current which flows in the conductors is everywhere the same. In the case of two stacked cross conductors, this gives an I2R heating essentially four times the heating across the strip width as compared to that along the strip edges, since heat is proportional to current squared.
In preferred embodiments of the invention, the induced current across the strip is essentially an integral multiple of that along the strip edges, with a preferred integer being two. The qualification xe2x80x9cessentiallyxe2x80x9d is used, because, in practice, some departure from integral multiple may be experienced, for instance because one conductor in a vertical stack of conductors will be farther from the strip than the other, or because one leg of a split-return inductor may carry slightly more current than the other. As implied, the qualification xe2x80x9cverticalxe2x80x9d is for the case of a strip in the horizontal plane; more generally, the departure will be for the case where the stacking is perpendicular to the plane of the strip.
The term xe2x80x9cstripxe2x80x9d is used generically herein and intended to cover elongated material in general, such as sheet, strip, plate, and cloth. Preferred, however, is material whose thickness is within the depth of current penetration d as defined in the article cited above in the BACKGROUND ART.